Let me put it otherwise:
x86 instructionset based uC's are overkill for simple embedded systems.
Ofcourse you can use a 'small' x86 uC but it would use more energy without using even half it's capability.
With that, the x86 instruction set is actually highly inefficient. In the past I have worked with a large number of different architectures (Sun Sparc, DEC Alpha, Power PC) and all of them are faster in the applications written for them.
The reason that x86has become the standard and so popular is that it is capable of doing a broad variety of things at quite some speed.
An example, I owned a DEC Alpha 166mhz workstation, with 512MB RAM.
Next to that I had a Pentium 3, 650MHZ with 1GB of RAM.
Booting Windows NT4 on both, switched on at the same time.
The Alpha would boot nearly twice as fast.
But when both where started up and I would run an application like Office, the P3 would blow it's socks off.
Same goes with the ARM cores that are widely used in tablets, smartphones, etc. They are efficient in some tasks, simpler tasks, but when it comes to running full blown systems an x86 system will always be better in use.
But don't try running them off of a small (phone sized) battery, it will drain it in a heartbeat.
Yes it is possible using them like we do with the AVR chips but don't go looking at efficiency